The invention relates to bodies with reversibly variable temperature-dependent light absorbence. These are suited for determining temperature, measuring temperature, and temperature data like, for example, frost warning devices, slippery ice warning devices, devices for absorbing radiation of the sun, also for greenhouses, industrial structures, office and dwelling space, vehicles, etc., as well as for temperature indication in technical devices and apparatus.
Bodies with reversibly variable temperature-dependent permeability to light are known from German Pat. No. 1,244,391 and German published patent application No. 2,154,042, which bodies consist of reversibly thermocoagulable synthetic substances, hydrate salts and, if necessary, water, or of hydrated polymers and/or copolymers of n-vinyl lactamens. In such bodies, the hydrate salts or hydrated synthetic substances give off water when a specific temperature is exceeded, which water is dispersed in small droplets in the synthetic substance and thus produces turbidity of the body. The disadvantage of such bodies with reversible temperature dependence resides in the fact that they exhibit variation in light absorbence only upon being heated to relatively high temperatures; that always a transition from a transparent state to an opaque state is possible only in one direction, that the reversibility of variation in light absorption is dependent on atmospheric moisture; and that no sharp sudden change in transparency occurs.
Further, from German published patent application No. 1,812,319 is known a reflector for slippery ice warning devices which consists of a fluid in a capsule, the congelation point of this fluid being so adjusted that it congeals densely above the freezing point of water and thus loses permeability to light. Such reflectors are relatively expensive and are not suited to be installed on street guide-posts. They are susceptible to breakage, since the capsules shatter in collisons or from wanton mishandling, and the fluid can run out; and finally, they have the disadvantage that because of their nature they are not suited to keep the frost warning indicators invisible in non-dangerous temperature ranges and make them visible only in the warning range.
The problem forming the basis of the invention, consequently, resided in acquiring new bodies with reversibly variable temperature-dependent light absorbence in which, as required, a transition from transparent to opaque or from opaque to transparent can be attained and the variability in light absorbence to almost any desired temperatures. A special problem resides in creating such bodies with reversibly variable temperature-dependent light absorbence which can be used as frost-indicating devices or slippery ice warning devices and change from opaque to transparent in the vicinity of the freezing point of water and thus, in case of frost or slippery ice, permit warning symbols, become visible or, if they themselves are formed as warning symbols, become visible by reason of turbidity. A further problem resides in obtaining such bodies in which the transition from transparent to opaque or vice versa is as sharp as possible; which are neither dependent on atmospheric conditions like atmospheric moisture nor susceptible to destruction; and which can be produced and installed as simply as possible and in abundance. Still a further problem resides in being able to give such bodies a preferred form, such as that of films (or foils, or sheets).